There is a need for such so-called single photon sources for use in optical quantum cryptography where, for example, the security key for an encryption algorithm is delivered by a stream of single photons which are regularly spaced in fine. It is essential for the security of this technique that each bit is encoded on just a single photon. This is because an eavesdropper tying to intercept the key will be forced to measure and thereby alter some photons during reading of the communication. Therefore, the intended recipient of the key can tell if the key has been intercepted.
Such a source is also useful as a low-noise source for optical imaging, spectroscopy, laser ranging and metrology. Normal light sources suffer from random fluctuations in the photon emission rate at low intensities due to shot noise. This noise limits the sensitivity of many optical techniques where single photons are detected. A single photon source which produces photons at regular time intervals has a reduced shot noise.
Previously, single photon sources have been envisaged by strongly attenuating the emission from a pulsed laser source. Such a source is shown in FIG. 29. The optical beam 201 is fed through an attenuator 203. The attenuator is configured so that the average number of photons transmitted is about 0.1. Since single photons are indivisible, his means that about 10% of the periods will contain a single photon, while 90% of them are empty.
This method has two serious drawbacks. First, there is still a small probability of finding two photons within one period. A significant number of these two photon emissions would seriously impinge on the security of the device for quantum cryptography. The second problem is that most of the periods are empty and hence carry no information. Thus, the time which it takes to send the key is increased, and also, the maximum distance over which it can be transmitted is limited. The distance limitation problem arises from the fact that optical quantum cryptography is only effective when the rate of detected photons is much higher than the ‘dark count’ rate of the detector. Therefore, a system with a high emission rate source can tolerate more transmission loss, and therefore a longer transmission distance medium before being overwhelmed by the detector dark count.
A single photon emitter is also being proposed by J. Kim et al in Nature, 397, p 500 (1999). The device proposed here utilises an etched single quantum dot. Single electrons and single holes are injected into the etched structure for recombination. The structure suffers from difficulties in injecting a predetermined number of both electrons and holes into the dot for recombination. Also, the proposed fabrication mode is awkward. Also, the method described of forming the quantum dot by etching produces a large number of non-radiative centres, which drastically reduces the emission efficiency of the device and leads to most of the injected electron-hole pairs being lost through non-radiative recombination.